1. Field of the Invention
The present invention relates to a surface plasmon resonance sensor device to be used for, for example, biosensors, optical sensors, gas sensors, concentration sensors, and ion sensors.
2. Description of the Related Art
Various sensors making use of surface plasmon resonance such as biosensors, optical sensors, gas sensors, concentration sensors and ion sensors have been proposed in recent years.
Surface plasmon resonance is a phenomenon in which—when a thin metal film having a nanometer order thickness and formed over one surface of a glass prism is exposed to an incident light from the reverse side of the thin metal film under total reflection conditions—an evanescent wave generated along the surface of the thin metal film reaches the opposite side of the thin metal film and resonantly excites surface plasmon which has a wave number equal to that of the evanescent wave. The thin metal film may be a fine-metal-particle thin film composed of fine metal particles distributed uniformly at regular intervals. In this case, surface plasmon resonantly excited on the surface of fine metal particles is sometimes called “localized surface plasmon”.
The wave number of the evanescent wave varies, depending on an incident angle of the incident light so that resonance excitation of the surface plasmon can be forced on the opposite side of the thin metal film by controlling the incident angle. The incident light, on the other hand, decreases its intensity (decrease in reflectance) because it consumes its energy when the evanescent wave causes resonance excitation of the surface plasmon. The angle at which the maximum decrease in intensity occurs is called “resonance angle” and it has a direct relation with the wave number of the surface plasmon. The wave number of the surface plasmon which is resonantly excited changes depending on the properties of the surface of the thin metal film on which the surface plasmon is excited.
It therefore becomes possible to detect properties, changes or the like of a substance on the surface of the thin metal film by determining the resonance angle from the angular distribution of the reflected light intensity and grasping a change in the resonance angle. It is also possible to detect the properties, change or the like of a substance on the surface of the thin metal film by grasping a change in the absorption spectrum or reflection intensity of a reflected light instead of a change in the resonance angle. The above-described various sensors can be manufactured by applying thereto the above-described principle.
As sensors utilizing surface plasmon resonance, those obtained by sharpening a tip portion of a light guiding core of an optical fiber in a light guiding direction and forming a thin metal film at the tip portion thus sharpened are conventionally known (refer to, for example, Japanese Patent Laid-Open No. 2001-165852).
In the above-described sensor, a predetermined light incident angle necessary for the surface plasmon resonance is set between a measuring plane, which has been obtained by sharpening the tip portion of a light guiding core of an optical fiber in a light guiding direction, and a light guiding direction by the light guiding core. In this sensor, when a light beam such as laser light irradiated from a light source is guided by the optical fiber to the measuring plane at the sharpened tip portion of the optical fiber, the incident light is reflected, depending on a substance to be measured. The properties, changes or the like of the substance are measured by detecting a change in the intensity of the light reflected depending on the substance to be measured.
The above-described method of grasping a change in the resonance angle and thereby measuring the properties, changes or the like of a substance requires a change in the angle of the incident light so that the apparatus used therefor becomes large and expensive. When the above-described sensor using the sharpened tip portion of a light guiding core of an optical fiber as a measuring plane is employed, on the other hand, an apparatus used therefor can be simplified because an incident light is guided to the measuring plane via the light conducting core and at the same time, a change in the intensity of a light reflected from the measuring plane may be detected by introducing the reflected light into a detector via the light conducting core.
The conventional sensors have however the inconvenience that since the tip portion of a light guiding core of an optical fiber sharpened along the light guiding direction is used as a measuring plane, an area effective for the measurement is limited, leading to difficulty in attaining sufficient measurement accuracy.